I saw truing-by-banging done pretty expertly at Burning Man, when a friend pranged her rear wheel pretty badly. Took it to one of the fix-it camps, where one of the guys told me he used to race downhill and did this kind of repair pretty frequently.

He held it with the bend down and away, and whacked it really hard against a workbench a few times. That was enough to make the wheel usable again.